Buck Converter Guidance

[elthanor]

Greetings,
I am new to this forum and i would like some assistance in a problem that i face.Using Proteus and reading through the internet i have created a Buck converter in order to understand how it works and after that use it in a project.The problem is that even though i think that i have done everything right the circuit refuses to work properly.
Here is an image:


Through the drain of the mos there are 10V and the settings i have for the generator are:


In normal circumstances this should give approximately 5V but instead the output is 10V as well.
Here is the oscilloscope view:


I am deeply frustrated as this is the simplest circuit for a Buck converter and i cant get it right even after spending a whole day searching for the mistake.Any help would be appreciated.
Thanks in advance

 

[FvM]

I presume that you don't yet fully understand how a buck converter works, otherwise you won't be suprised that the output voltage is not 5 V. To give a small hint, there's a point of discontinuous conduction (DCM) versus continuous conduction mode (CCM) operation. The simplidied duty cycle to output voltage translation is only valid in CCM, which isn't achieved for the given circuit parameters.

But instead of further studying literature you should primarly use the power of the simulation tool to look for the interesting details of circuit operation, which are presently ignored in your oscilloscope view. Which voltage waveform do you expect for the switching (source) node, how about transistor, diode and inductor current? Did you try to change gate drive frequency or duty cycle? What did you do during the "whole day"? Drinking coffee?

As a side remark, the presently implemented gate drive method can only work under very limited conditions, it's already touching the maximum Vgs limited. A real buck converter will usually implement some kind of level translation to drive the gate with a defined voltage between gate and source rather than gate and circuit ground.

 

[elthanor]

Thank you very much for the info.It really helped me a lot and i actually got it to work correctly at last.In my project i have a PWM pulse from a microprocessor which is 5v.In order to achieve a positive Vgs i thought summing the PWM pulse with the mosfet's source voltage with an operational amplifier.Is something like this feasible?And if it is how can i power the opamp autonomously without adding another power source except the one i have in the mosfet's drain.
Thanks in advance!